The present invention relates to a plastic film formed from polypropylene or the like that has a good running characteristic and to a production method for the same.
Due to their excellent properties, plastic films are widely used for industrial purposes, for example, capacitors. As greater quantities of plastic films are being demanded, speedups in all plastic film-processing steps are lately being pursued.
However, as the processing speed is increased, meandering, deviation, wrinkling and the like during film wind-up and the like have become more likely to occur, and have become factors that impede increases in productivity.
Therefore, a high-speed processing characteristic is required in the plastic film itself. Winding-up characteristic, running characteristic and the like are now important quality requirements.
Conventionally, various proposals have been made to prevent deviation and wrinkling of a plastic film during the slitting, the winding-up and the like of the film. For example, Japanese Patent Application Laid-Open No. SHO 58-16415 discloses that the wetting tension of a non-vapor-deposited surface is specified. Japanese Patent Application Laid-Open No. SHO 60-183449 discloses that the coefficient of static friction of a vapor-deposited film is specified.
Furthermore, Japanese Patent Application Laid-Open No. SHO 62-20137, as for example, proposes a method in which, in a film vapor deposition step, a film is uniformly electrified immediately after the film is unwound, and the film is diselectrified immediately before it is wound up.
However, in the case of polypropylene films, which are relatively apt to have winding deviation, wrinkling or blocking among the plastic films, the aforementioned conventional arts are still insufficient, for example, with regard to the running characteristic or the winding-up characteristic under a high-speed condition of at least 400 m/min.
For example, in the art described in Japanese Patent Application Laid-Open No. SHO 58-16415, both surfaces of a film are subjected to a corona treating process, so that a drawback exists; for example, wrinkling is likely to occur and, therefore, films may block each other. Furthermore, merely specifying the coefficient of static friction of a vapor-deposited film as described in Japanese Patent Application Laid-Open No. SHO 60-183449 is not adequate for the running characteristic in the atmosphere. In either proposal, the drawback is particularly remarkable in, for example, a case where a film has a small thickness of 10 .mu.m or less, a case where the rigidity of a film is low in comparison with the wind-up tension due to the material of the film, etc.
Furthermore, a method of uniformly electrifying a film immediately after unwinding the film, and diselectrifying the film immediately before winding it up, as described in Japanese Patent Application Laid-Open No. SHO 62-20137, can prevent slippage between a cylindrical can and a guide roller or the like during the film-running and can achieve an improvement with regard to meandering during the running. However, due to the diselectrification immediately before the wind-up, the method is insufficient as a measure for preventing a winding deviation.